In recent years, the electrophotography has been applied to copying machines as well as various printers since they can give images with high qualities without delay. As a photoreceptor which plays an important role in the electrophotography, the photoreceptor comprising an inorganic photoconductive material such as selenium, arsenic-selenium alloy, cadmium sulfide, zinc oxide and the like has been used. More recently, the photoreceptor comprising an organic photoconductive material was proposed. The latter has the advantages which is not a pollutant and which has a film-formability and a shapability.
As one of the organic photoreceptors, the so-called "laminated-type photoreceptor" in which a charge generation layer, the thickness of which is usually about 0.5 .mu.m, and a charge transport layer, the thickness of which is usually about 10 to 20 .mu.m, are successively laminated was developed. The laminated-type photoreceptor is increasingly interested in and is expected to be widely used in the near future because it has the following advantages:
(1) the photoreceptor having high sensitivity can be obtained by suitably selecting and combining the charge generation material and the charge transport material;
(2) the photoreceptor having high safety can be obtained because the charge generation material and the charge transport material can be selected from a wide range of the materials; and
(3) the photoreceptor can be prepared by simple coating and thus it can be prepared with low costs.
In general, a photosensitive layer comprising the charge generation layer and the charge transport layer is formed on a conductive base according to any one of the known methods such as a dip coating method, a spray method, a wire bar method, a blade method, a roller method, a curtain coater method and so on. When the conductive base is an endless pipe, the dip coating method wherein an object to be coated is dipped in a vessel containing a coating solution followed by lifting the object from the surface of the coating solution at a constant speed is usually and preferably employed because it can give a coated film with an uniform thickness relatively easily.
The prior laminated-type photoreceptors are very poor in durability when compared with the inorganic photoreceptors so as to limit their application.
One important cause of such a poor durability is that the thickness of the charge transport layer reduces by being subjected to the abrasion during the cleaning step of the electrophotographic process. The reduction in thickness of the charge transport layer is accompanied by the lowering of the charged potential and thus the lowering of the contrast on the resultant images. As one of the effective means for preventing the reduction in thickness of the charge transport layer, it is proposed to increase the thickness of the charge transport layer so as to prevent the change of the charged potential.
The approach of increasing the thickness of the charge transport layer has two problems. Firstly, the charge transport layer with the increased and uniform thickness cannot be obtained according to the conventional dip coating method because a large volume of the coating solution drop down and the coating cannot be conducted at the suitable speed. For effectively forming the charge transport layer with the increased and uniform thickness according to the dip coating method, the use of the low molecular weight polymer as a binder resin so as to prepare the coating solution having the high solid concentration and the reduced viscosity is considered. However, the abrasion resistance of the charge transport layer is impaired when this coating solution is used, and as the result, the advantage effected by increasing the thickness of the charge transport layer will be compensated.
Secondly, when the thickness of the charge transport layer increases, the photoreceptor has the low optical responsiveness. Because, the increase of the thickness of the charge transport layer weakens the electric field strength which affects the mobility of carriers and the optical responsiveness of the photoreceptor.
An object of the present invention is to provide the electrophotographic photoreceptor having the excellent durability and the excellent electric properties for long period, which can be easily and efficiently prepared.
The present inventors found that the above object of the present invention can be achieved by forming the thicker charge transport layer with the coating solution containing a specific polymer as the binder resin according to the dip coating method.